VALVE JOB #3: PREVENTING BACKFLOW

It's only one...

One way to go
One way to go
One way to go
One way to go

("One Way to Go," by The Verve, 1993)

THE THIRD JOB OF VALVES

To prevent backflow, there is a Check Valve piped into the (vertical) Discharge Line of each of the four Centrifugal Pumps shown.

Brilliant PTOA Readers and Students ... meaning those who read the PTOA Segments in the intended, sequential order ... recently learned in PTOA Segment #252 about Valves with the job of starting and stopping the PV Flowrate. Valves with the job of throttling and regulating the PV Flowrate were featured in PTOA Segment #253.

This PTOA Segment features the third job of Valves:

Preventing backflow is the job of Check Valves.

The arrow in the ISA symbol for a Check Valve points in the direction of the intended, normal pathway.

The nearby graphic depicts the ISA symbol for a Check Valve. On the P&ID, the arrow points in the direction of intended flow.

VALVE FUNCTION #3:
VALVES THAT PREVENT BACKFLOW

Check Valves (Swing Check Valve, Lift Check Valve, Ball Check Valve)

Top: A Dual-Plate Swing Check Valve (the same type of check valve as shown on the nearby gif). Bottom: A Swing-Check Valve shown in the closed position and imagining where the disc would be in the open position.

The job of the Check Valve is to prevent the PV Flowrate from flowing in the opposite direction than the intended direction.

This gif illustrates how a Check Valve works. The normal flow direction is from the right to the left. When flow reverses ... flowing left to right ... the Check Valve closes.

Check Valves are open when the PV Flowrate is going in the intended direction. Check Valves close when the PV Flowrate reverses direction and starts flowing backward.

Brilliant PTOA Readers and Students already know that the PV Flowrate will only flow in the direction of higher pressure to lower pressure. Therefore, it seems logical to just make sure the ΔP is always favorable to keep the PV Flowrate flowing in the intended, desired direction.

Still ... stuff happens.

The typical Centrifugal Pump piping plan includes a Check Valve on the Discharge Line.

Centrifugal Pumps were featured in PTOA Segments #163 through PTOA Segment #176. Centrifugal Compressors were featured in PTOA Segment #219 through PTOA Segment #221.Centrifugal Pumps and Centrifugal Compressors can fail unexpectedly, and the resulting backflow would damage their Impeller blades.

Check Valves will be on the discharge line of every Centrifugal Pump or Compressor to protect Impeller Vanes/Blades from backflow damage.

Backflow can even occur during a planned shutdown or startup for a Centrifugal Compressor or Gas Turbine.

For the reasons listed above, a Check Valve will be piped into the Discharge Line of every Centrifugal Pump and Centrifugal Compressor.

Swing Check Valves

This Swing Check Valve has a circular gate which normal flow pushes open and backflow closes shut. Normal flow for this check valve is from the left to the right side of the gate.

The flow element in a Swing Check Valve is a Disc that can swing open or closed because it is on a Hinge. The normal, intentional flow path keeps the Disc open.

Should the PV Flowrate reverse and start to backflow, the Velocity Factor of the PV Flowrate (see PTOA Segment #235) and the PV Pressure of the fluid in the process line work together to move the Disc into the closed position. The Disc is hefty; hence the Swing Check Valve only works well when the PV Flowrate is fast.

The Disc would slam shut and cause hydraulic shock if it were not pivoted from the Hinge.

The Body of a Swing Check Valve is similar to the full ported body of a Gate Valve. The metallic Seat is designed to fit the full diameter of the pipe, aka the Seat is "full-ported."

Lift Check Valves

Recall that Swing Check Valves work better to prevent backflow in process services characterized by consistently high PV Flowrates.

Lift Check Valves work better preventing backflow when the PV Flowrate fluctuates.

The nearby graphic compares a Swing Check Valve to a Lift Check Valve. The flow element within a Lift Check Valve is a Plug connected to a Stem (ergo, a little piston) that has positional guidance provided by the tension of a spring.

Normal flow into the Lift Check Valve is from right to left. The flow into the Lift Check Valve enters the lower part of the Valve Body and pushes the Plug upward. Backflow pushes the Plug into the closed position.

Note how the Body of the Lift Check Valve is similar to the Body of a Globe Valve featured in PTOA Segment #253.

Incoming flow must flow to the bottom of the Lift Check Valve. The force of the fluid that is flowing in the intended direction must lift the Plug upward to allow flow to continue to the upper part of the Lift Check Valve and thence out of the Lift Check Valve.

Lift Check Valves are made from modified Globe Valves.

Reversal of flow causes the Plug assembly to be pushed downward, thus blocking backflow.

Ball Check Valves

Both the Swing Check Valve and the Lift Check Valve have more moving parts than the Ball Check Valve.

And the problem with any moving part is that it eventually wears out.

Refresh your browser if necessary to see this Ball Check Valve gif.

Ball Check Valves rely on a Ball flow element to block backflow. The Ball may or may not be held in place by a Spring, but there is no Disc or Hinge or Stem, etc. Ergo, Ball Check Valves are more maintenance free than either Swing Check Valves or Lift Check Valves.

The next PTOA Segment features the final job of Valves, managing and safely getting rid of excessive PV Pressure.

TAKE HOME MESSAGES: Check Valves prevent backflow. Backflow prevention is needed at the Discharge of Centrifugal Pumps and Compressors since backflow would damage Impellers.

Swing Check Valves must be used with high PV Flowrates because the Velocity Factor of the PV Flowrate keeps the Disc element in the open position. Swing Check Valves are full ported and can be made by modifying the Body of a Gate Valve.
Lift Check Valves are used with PV Flowrates that fluctuate. Swing Check Valves direct flow to the lower part of the Check Valve; the Velocity Factor of the PV Flowrate must push a plug upward to allow the flow to continue to the Valve Outlet. Lift Check Valves are made from modified Globe Valves.
Ball Check Valves have less moving parts than Swing Check Valves or Ball Check Valves.

PTOA PV FLOWRATE FOCUS STUDY AREA
PIPING NETWORK HARDWARE

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